The present invention is directed to the field of materials processing using lasers and, more particularly, to a method and apparatus for monitoring laser weld quality via plasma size measurements.
High power lasers are commonly used for materials working processes such as laser welding, cutting, drilling and heat treating. These processes provide a number of important advantages over conventional welding processes including enhanced speed, consistency and weld quality.
During laser materials working processes, the laser beam impinges on a workpiece, which becomes heated and eventually melts and then vaporizes. This vapor and the surrounding gases are ionized by the extreme heat and form a plasma plume between the laser and the workpiece. Weld quality is affected by the instability of the plasma formation and by instabilities in process operating conditions such as fluctuations in the laser power and shield gas flow, and by workpiece defects such as weld zone contamination and physical deformation.
As the use of laser materials working processes increases in industry, the need for accurate in-process techniques for monitoring process quality increases as well. In-process techniques provide important advantages as compared to post-process non-destructive quality control techniques such as x-ray and ultrasonic analysis, and visual inspection; and destructive quality control techniques such as metallography. Such post-process techniques tend to be labor intensive and tedious, and do not enable real time monitoring and control of the laser processing.
U.S. Pat. No. 5,360,960 discloses a laser process monitoring system by counting the number of violations each time the plasma light intensity violates one of the predetermined plurality of light limits. Known in-process techniques for monitoring laser materials working processes are not fully satisfactory. Particularly, known techniques can falsely reject good parts (type I error) or not reject bad parts (type II error). Type I errors are detrimental in terms of economic cost. Type II errors can be especially important, for example, in laser welded parts that require high weld quality at certain critical welds. The failure to detect bad welds can result in potentially dangerous parts being placed into components.
Thus, there is a need for a method and apparatus for monitoring laser materials working processes that can be used (i) in-process; (ii) to distinguish between good and bad welds and, thus, reduce the rate of type I and type II errors; and (iii) in various laser-material processes.